Field emission ion source



Oct. 8, 1957 R G, HERB 2,809,314

FIEL EMISSION ION SOURCE Filed Jan 27, 1956 2 Sheets-Sheet l Oct. 8;1957 R. G. HERB FIELD' EMISSION 10N SOURCE 2 Sheets-Sheet 2 Filed Jan.27, 1956 [IIL Patented Get. 8, 1957 man ntvirssioN roN Sonaca RaymondLvG. Herb, Madison, `Wis., assigner to High Voltage EngineeringCorporation, Camhridge, lvass., a corporation of MassachusettsApplication January 27, 1956, Serial No. 561,818

19 Claims. (Cl. 313-63) This invention relates to the generation oi ionsby emission from metallic points under the influence of very highelectric fields, and in particular to an ion source comprising at leastone hollow member having a point, such as a hollow needle, the interiorof which is supplied with the gas to be ionized, and high-voltage meansfor producing a very high electric ield at the point of the needle, sothat gas which permeates through the needle is ionized by the electriciield after it arrives at the point of the needle.

While the invention is not limited to any particular application, an ionsource embodying the invention is especially yadapted for use inparticle accelerators, since it provides an ion beam which can be veryprecisely focussed and collimated, and permits reduction in the size ofaccelerating tube apertures and production of a better tube vacuum.These improvements, in turn, lead to a reduction of the long tubeeilect, yand better shielding of the beam from static inucnces in theaccelerating tube.

An ion source constructed in accordance with the invention provides asubstantially monoencrgetic ion beam. In the conventional ion source,the ions are created by means of a discharge through a gas, and thevoltages which are necessary to produce the discharge cause ions to beaccelerated by varying amounts up to the maximum voltage drop in the ionsource. F or example, most capillary-arc sources and radio-frequencysources have an energy spread of more than l() electron volts. in theion source of the invention, the only energy spread is that due to thethermal agitation of the gas at the point of the needle, which may be aslow as one-iortieth of an electron volt. Thus the invention provides abeam of ions in which the energy spread is very small and comparable tothat encountered in an electron source.

An ion source constructed in accordance with the invention is capable ofproviding a substantial well-collimated ion beam, because the degree ofrandom distribution of the position from which and the direction inwhich the ions are emitted is extremely small. That is to say, each ionin the beam originates from some point within a very smal area, and eachstarts out in a direction within a very small solid angle. Substantialion currents are possible, despite the smallness of the emitting area,owing to the high luminosity of the ion source of the invention, where"l` minosity is a measure of the number of ions emitted "ci unit area ofthe emitting surface. For example, a capi 3e source may provide an ionfrom an area of about s on the order or" l sity is l square mi limeter,and cui milliampere; so that the lumi per square centimeter.

in the ion source of the invention may be as low as 6.3 X-10 square cencurrent as low as l microampere would of 1600 amperes per squarecentimeter.

The angular limites of the emitted ions is proportional to the ratio ofthe magnitude of the random velocities possessed by the ions in thesource to the gradient of the accelerating voltage at the source. Aspreviously noted, the random velocities in the ion source of theinvention are very small. Moreover, the accelerating voltage gradient atthe source is on the order of 108 Volts per centimeter, which farexceeds the gradients in conventional sources. The high initial gradientfurther assists in providing a well-collimated beam by virtue of thefact that it reduces the eect of space charge.

It is possible to pulse an ion source constructed in accordance with theinvention, and by the application of short electric pulses relativelyhigh instantaneous currents may be realized. A simple calculation showsthat there are enough atoms at the point of the needle to provide ashort, high current pulse. Assuming the gas layer on the point to be 1atom thick, and assuming that the point is a hemisphere of radius 5x10-5centimeter, if the pulse interval is 10-9 second, the current can be ofthe order of 10 milliamperes. Moreover, the rapid replenishment of thegas at the point permits a high pulse repetition rate.

As will appear from the following detailed description thereof, an ionsource constructed in accordance with the invention is mechanicallysimple. Moreover, the gas flow required to provide a given proton llowis less than that required in conventional ion sources.

The invention may best be understood from the following detaileddescription thereof, having reference to the accompanying drawings, inwhich:

Fig. l is a view in central section of one embodiment of the invention;

Fig. 2 is an enlarged view of the ion emiter shown in Fig. l;

Fig. 3 is a diagram showing the electrical circuit associated with theapparatus of Fig. l;

Fig. 4 is a view similar to that oi Fig. 2 and showing .one modificationof the apparatus or Fig. 2;

Fig. 5 is a View similar to that of Fig. 2 and showing anothermodiiication of the apparatus of Fig. 2; and

Fig. 6 is a view similar to that or Fig. 2 and showing apparatus forproducing a pulse ion beam in accordance with the invention.

The phenomenon of field emission, by which appreciable amounts ofcurrent can be drawn from metals even at low temperaturesby theapplication of very high electric elds at the surface, has been knownsince 1897 in connection with the emission of electrons. Classicaltheory could give no reasonable mechanism which explained fieldemission. In the quantum mechanical picture, some of the multitude ofconduction electrons penetrate the potential barrier of the metalsurface when this barrier is narrowed by a strong applied iield.

Recently the phenomenon of eld emission has been observed in connectionwith positive ions. Field emission of ions dii-ters from eld emission ofelectrons in that the metal at whose surface the high electric tield iscreated does not ordinarily possess ion reservoir comparable to the vastmultitude of 'hilo electrons which are available for electron emission.Consequently, the principal problem in causing the held emission of ionsfrom a metallic point is the provision at the point of atoms ormolecules which will be ionized by the high electric iield. The problemis augmented in the case of an ion source suitable for use in particleaccelerators and similar devices by the necessity for continuousreplenishment of the ionizable atoms or molecules at the point. Whilelimited currents have been achieved with metal points of specialmaterial and with metals in which hydrogen gas has been absorbed, theconstruction of a practical ion source has been thwarted by inadequatereplenish ment ofthe ionizable material at the emitter point.

The invention provides the, necessary replenishment of ionizablematerial at the emitter point by using a hollow needle filled with gasunder high pressure, and, in some cases, by also heating the hollowneedle. The invention makes use of two phenomena: first, the ability ofcertain gases to permeate certain metals under certain conditions;second, the fact that under certain circumstances the gas which haspermeated the metal to the surface thereof will remain there, forming alayer of molecules which can ow along the surface of the metal.Accordingly, the gas which is within the hollow needle at high pressurepermeates through the walls of the needle to the outer surface thereof.One might suppose that the flow of gas to the point of the needle wouldbe very limited, owing to the fact that the thickness of the needle inthe immediate vicinity of the point must be relatively thick in order toprovide the necessary mechanical support for the point. However, owingto the second phenomenon mentioned above, the gas need not ilow throughthis thick part of the needle Wall in order to reach the point. needlewall, forms a layer of molecules on the outer surface of the needle, andthen ows along the outer surface of the needle to the point. In thisway, the gas which is pulled off the emitter point by an intenseelectric field is replenished by the surface flow, the magnitude ofwhich is sufficient to replenish gas at the point at the high emissionrates required for practical use.

f course, the gas selected and the material of which the needle or otherhollow pointed member is composed must be such that the gas willpermeate through the needle. Fortunately, the gas which is mostfrequently used in ion sources for particle accelerators is also a Vgaswhich is capable of permeating most metals. This gas is hydrogen,including its isotopes deuterium and tritium; and in the claims the termisotopes of hydrogen includes hydrogen, deuterium and tritium. Whilemost metals are permeable to isotopes of hydrogen, palladium isparticularly so and for this reason is a preferred material for thehollow needle. However, palladium has low tensile strength, and so insome cases it may be desirable to use a stronger metal, such as iron ornickel, in view of the large force exerted on the needle by the intenseelectric eld. Copper has a relatively low permeability to isotopes ofhydrogen, and so those portions of the gas enclosure through which thegas is conveyed 'to the hollow needle may conveniently be made ofcopper.

Referring to the drawings, and first to Fig. l thereof, the ion sourcetherein Vshown is constructed in accordance with the invention andcomprises an evacuated chamber 1 which contains a needle assembly 2 andan accelerating f lens system 3, 4, 5. The chamber 1 is evacuated bymeans of a suitable pump (not shown). Y

The chamber 1 includes an insulating column, consisting of insulatingrings 6 separated by and sealed to apertured thin electrode disks 7,Vand a lead-in flange 8. Metallic rings 9 it over the sharp edges of theexposed Instead, it flows through the thin parts of the electrode disks7 Vto prevent sparking and corona discharge from theseV sharp edges.Resistors (not shown) may be provided between adjacent electrode disks-7to provide the desired distribution of potential along the column and toprevent formation of any high, localized potential gradients which mightcause breakdown.

T he needle assembly 2 is shown in detail in Fig. 2. VA copper tube 10,one-quarter inch in diameter, is silver soldered into the lead-in ange 8(Fig. l). The end of the copper tube 1f; is drilled out slightly, and acopper snout 11 is inserted into the resultant annular recess 12 in thecopper tube 10 and is silverv soldered in place. Y A hollow needle 13 issilver soldered in the snout 11, as shown in Fig. 2. lt should beemphasized that the invention is not limited to the particular needleassembly shown, but includes any suitable hollow member having a point.For example, the needle assembly 2 shown in Fig. 2 may be modilied bymachining the tube 10 and the 4 Y snout 11 out of a single copper rod,thereby reducing the number of silver-soldered joints.

In accordance with the invention, gas is fed at high pressure into theneedle assembly 2 from a suitable gas source 14. The needle 13 consistsof a material which is permeable to the gas from the gas source 14; andso, as appears from Fig. 2, the wall through which the gas will flow isthat portion of the needle 13 which extends from the extremity of thesnout 11 nearly to the point 15 of the needle 13. For example, if theneedle 13 is a rod one-sixteenth inch in diameter and two centimeterslong, and if a hole one-thirty-second of an inch in diameter is drillednearly the entire length of the rod, as shown in Fig. 2, the needle 13will have an inner area of about one-half square centimeter available asa permeation surface, while the wall thickness is approximately .045.

centimeter. Y

The high pressure alone may be sullcient to cause the gas to permeatethrough the wall of the needle 13 so as to reach the outer surface ofthe needle 13 and, owing along the outer surface, arrive at the emitterpoint 15. If not, the temperature of the needle 13 may be increased toassist in such permeation by means of a heater coil 16, which isenergized by a suitable power supply 17. If the needle 13 is to beraised to hightemperature, the heater coil 16 may be an electron emitterwhich bombards the needle 13 with electrons so as to raise itstemperature by a large amount.

lf the needle 13 of the foregoing example is made of palladium, and ifhydrogen is introduced into the needle 13 at a temperature of 300 K. anda pressure of 1500 pounds per square inch in excess of the pressureprevailing outside the needle 13, the flow rate through a wall may becalculated to be V1 l05 cubic centimeters per second. This flow ratecorresponds to a current of 100 microamperes. lf the needle 13 is madeof iron, and if hydrogen is introduced into the needle 13 Vat atemperature of 500 K. and a pressure of 1500 pounds per square inch inexcess of the pressure prevailing outside the needle 13, the ow rate maybe calculated to be 2X109 cubic centimeters per second. This flow ratecorresponds to a current of .02 microampere. Thus a palladium pointeasily supplies the necessary flow rates, While an iron point must beheated to raise its permeability.

Gas arriving at the emitter point 15 is ionized by a very high electricfield which is produced at the point 15 by impressing a high-voltagepotential difference between the point 15 and the lens system 3, 4, 5.The electric field at the point 15 increases as the potential differencebetween the point 15 and the lens system 3, 4, 5 increases, and alsoincreases as the radius ofcurvature of the point 15 decreases. Themaximum size of the point 15 thus depends upon the electric fieldnecessary and the potentialV difference available.

Field strengths of (2 to 6) l08 volts per centimeter are necessary forproper operation of the ion source. An accurate calculation of thevoltage produced by a given emitter point can be made following a methodwhich assumes that the shape of the point can, for calculation purposes,be considered as a sphere on a cone.V However, a calculation assumingthe point to be a paraboloid will suliice Where only an approximatevalue of the field produced is desired. The latter assumption leads tothe formula: Y

rin-

where F is the electric eld in volts per centimeter, U is the appliedpotential in volts, r is the radius of the emitter, and l?. that of theextractor, both in centimeters. Calculations from this formula and fromcertain data indicate that with a maximum voltage of 35 kilovolts apoint Vradius of (2 to 3))(10-5 centimeters is required.

The general order of the magnitude of r and U may be calculated asfollows:

Since F must be of the order l08 volts per centimeter, r (incentimeters) must equal U (in kilovolts) times lO-S.

Fine points such as this have been produced in several ways. Among theseare mechanical grinding, chemical etching, oxidation in a flame, andelectrolytic etching. Of these methods electrolytic etching seems toproduce the smoothest surfaces with the least irregularities. Such asurface is desirable in the present invention primarily because of itsgreater mechanical strength. Iron points were successfully etched in amolar solution of potassium chlorate, dissolved in a 30 percent solutionof hydrochloric acid. A nickel helix about 2 inches in diameter was madeone electrode, and the emitter point the other. Sixty-cycle alternatingcurrent of about volts potential was used and is satisfactory. The pointwas etched for a short period of time, then inspected under an opticalmicroscope, then etched again. This etching was continued until theradius just passed the limit of resolution of the microscope. Thisplaces the radius of the point at approximately 3XlO5 centimeters. Foran accurate knowledge of the point, an electron micrograph must be made;although the general shape of the point can be determined throughobservation with a high power optical microscope. The points which wereetched were approximately paraboloids. The same methods can be used toetch palladium points.

Any suitable means may be used to create the necessary high electric eldat the point l5. In general, however, it will be desirable to focus theemitted ions into a beam, and in that event the electrode to which theions are attracted from the emitter point may comprise part or" asuitable lens system 3, 4, 5. The invention is not limited to anyparticular lens system; nor, indeed, is it necessary to the operation ofthe invention that a lens system be used at all. Merely by way ofexemplifying one possible embodiment of the invention, the lens system3, 4, 5, shown in Fig. l, is a tube type einzel lens, and comprisesthree lens electrodes 3, 4, and 5, which are constructed ofone-siXteenth-inch wall seamless steel tubing, and each of which ispolished and the ends carefully rounded. The electrodes 3, 4, 5 arefastened into stainless steel disks 1S thirty mils thick, which aresupported on the electrode disks 7 of the chamber 1.

The electric circuit for the lens system 3, 4, 5 is shown in Fig. 3.Referring thereto, electrodes 3 and 5 are grounded, and a high-voltagesupply 19 maintains the needle assembly 2 at a suitable potential, suchas 50 kilovolts, which may be positive or negative depending on Whetherthe ion source is to provide positive or negative ions, respectively.The middle electrode 4 is maintained at a potential between ground andthe potential of the needle assembly 2 by means of a potential divider2li. lf a pulsed ion beam is desired, the high-voltage supply 19 maycomprise a suitable circuit for producing highvoltage pulses at theneedle assembly 2; or, alternatively, any other suitable pulsing meansmay be employed. As hereinbefore stated, an ion source embodying theinvention is well adapted to the production of high-current ion pulsesof short duration.

he lens system 3, 4, 5 serves to focus the ions into a beam after theyleave the emitter point 15. The primary reasons for choosing a tube typeeinzel lens for this purpose were: (l) simplicity of construction-thelens itself is simple, and the voltage supply 19 which is used for theemitter point 15 can also serve for the lensY system 3, 4, 5; (2) if thelens voltage is a definite fraction of the emitter voltage, the focallength of the lens does not depend on the voltage applied to the emitterpoint. To provide the lens with a variable focal length, the voltagesupply for the lens is taken from the high-voltage supply i9 through thepotential divider 20. ln this system the focal length of the lens isdetermined by the setting ot the potentiometer 20. Once this setting hasbeen made the emitter voltage may be varied at will without destroyingthe focus of the beam.

The rst electrode 3 is equipped with an aperture 21, which serves bothas a limiting diaphragm defining the beam, and as an extractor for theemitter point 15. The aperture 2l shown in Fig. l is designed to permita sixtydegree cone of protons to enter the lens from an emitter point 15one centimeter distant.

In a uni-potential lens the dimensions of the center electrode 4 are theonly critical dimensions. The other two electrodes 3, 5 lare designed toserve the purpose of shielding the beam from any static charge whichmight build up on the insulating walls of the column of the chamber 1.Therefore the electrodes 3, 4, 5 are made reentrant by one-quarter inch,and are made long enough to cover all insulating surfaces exposed to thebeam.

A major application of the invention is its use as a positive-ion sourcefor particle accelerators, and especially as a source of protons anddeuterons for such particle accelerators. However, the invention is notlimited to such application, but includes lother applications where apoint source of ions is desired. For example, the invention may beembodied in a point source for negative ions, in which event theundesired emission of electrons from the metallic needle is minimized bythe layer of gas which covers the outer surface of the metal, ashereinbefore set forth. Any electrons which are emitted may easily beremoved from the negative-ion beam by conventional means, such ascausing the beam to pass through a magnetic tield strong enough todeflect the electrons yet not strong enough to attect the negative ionsappreciably. When the invention is embodied in a point source fornegative ions, the eld strength required at the point is 'only of theorder of l0I volts per centimeter, rather than the 108 volts percentimeter required for positive-ion operation.

As hereinbefore stated, the ion source of the invention comprises atleast one hollow member having a point, and in the embodiment of theinvention which is shown in Figs. l and 2 said hollow member having apoint comprises the needle assembly 2. Only part of the hollow memberneed be of a material permeable to the gas compressed within it, and inthe embodiment shown in Figs. l and 2 only the hollow needle 13 ispermeable to the gas. However, other constructions of the hollow memberhaving a point are possible without departing from the spirit and scopeof the invention, and two alternative constructions are shown in Figs. 4and 5.

Referring now to Fig. 4, therein is shown a needle assembly 2' which issimilar to the needle assembly 2 shown in Figs. l and 2, except that thetube 1l) and snout 1i terminate in a solid needle 13'. The solid needlei3 is easier to manufacture than the hollow needle 13 (Fig. 2), but thegas flow is reduced because the gas must permeate through more solidmaterial. Nevertheless, if the material of which the solid needle 13 iscomposed readily transmits the gas to be ionized, the construction shownin Fig. 4 may be used. For example, a solid needle 13 composed ofpalladium would transmit hydrogen sundciently readily to permit the useof the embodiment of the invention shown in Fig. 4. In the embodiment ofthe Vionized permeates through the material of the pointed member tothe-outer surface thereof, and then travels along such outer surface tothe point of the pointed member. For this reason, it is not necessarythat the point or tip of the pointed memberbe especially permeable tothe gas, and in some cases it may be desirable that the tip of thepointed member be composed of a material different from that of whichthe rest of the pointed member is composed. For example, palladium ishighly permeable to hydrogen, but it is soft. In accordance with theinvention, a pointed member composed of palladium may be provided with atip composed of harder material such as tungsten, since the material atthe tip need not be permeableV to the. gas. Referring to Fig. 5, thereinis shown a hollow needle 13 whose shaft 21 is composed of a material,such as palladium, which is permeable to the gas to be ionized, butwhose tip 22 is composed of a different material, such as tungsten, andmay be welded to the shaft 21 or otherwise aliixed thereto. in theembodiment of the invention shown in Fig. 5 the tip 22 holds its shapebetter and is less apt to be deformed, lasts longer, and is easier tomanufacture than point 15 of the needle 13 in the embodiment of theinvention shown in Fig. 2.

lt has hereinbefore been suggested that, if a pulsed ion beam isdesired, the high-voltage supply 19V (Fig. 3) may comprise a suitablecircuit for producing high-voltage pulses at the needle assembly 2.However, the highvoltage supply 19 must provide a voltage of the orderof 104 volts, and pulsing a voltage of this magnitude is a dimcult task.ln accordance with the invention a pulsed ion beam may be produced in amuch simpler manner by modifying the power supply 17 (Fig. l) so that itcreates a controllable potential dierence of the order of a few hundredvolts, or even less, between the heater coil 16 and the needle 13. Theneedle 13 is kept at a fixed potential,Y and the voltage'of the coil 16with respect to the needle 13 is pulsed by means of the power supply 17.As shown in Fig. l, the power supply 17 is at the high potential end ofthe ion source, and it need deliver pulses or" only a few hundred Voltsor less.

For improved pulse control, the embodiment of the invention shown inFig. 6 may be used. The needle aspulse-voltage power supply 17.'V'lhehigh-voltage supply 19 keeps the needle assembly 2 at a highpotential, such i as 25 ltilovolts, and the pulse-voltage power supply17 delivers voltage pulses of the order of hundreds of volts or lessbetween the plate 23 and the needle assembly 2. i' Assuming that theapparatus of Fig. 6 isdesigned to emit lpositive ions, the needleassembly 2 will be at a high positive potential, such as +25, kilovolts.As the potential ot the plate 23 becomes more positive than VLI-Zkilovolts, the tield pattern at the point 15 will become distorted insuch a way that fewer eld lines will end at the point 15, therebyreducing the iield strength and decreasing the ion emission from thepoint 15. Conversely, as the potential ofrthe plate 23 becomes lesspositive than +25 kilovoltsfion emission from the point 15 is increased.For example, field emission may be cut off when the potential of theplate 23 reaches +26 kilovolts. y in that event, l-kilovolt pulses fromthe power supply 17 would. suffice to produce a pulsed ion beam.

' ln general, the voltage output required of the power supply 17 will beon the order of hundreds ofvolts or less.

The effect of the plate 23 upon the ion emission from the point 15 isanalogous to the effect of the grid in a conventional triode on theelectron current to the plate `of the triode, since small variations inthe potential diierence between the needle assembly 2 and the plate 23change the 'eld strength at the point 15 a great deal, therebycontrolling the ion emission from the point 15. As the plate 23 is movedback further away from the point 15 than the position shown in Fig. 6,sensitivity is reduced. As the plate 23 is moved forward towards the tip15, sensitivity is increased. Sensitivity is greatest when the plate 23is in front of the tip 15, but in that event some bad effects might beencountered: for example, it might not be possible to attain thenecessary 108 volts per centimeter at the point 15 for positive-ionemission, or the necessary 107 volts per centimeter for negative-ionemission. Y

Having thus described the principles of the invention,

" Vtogether with several illustrative embodiments thereof,

it is to be understood that although specific terms are employed, theyare used in a generic and descriptive sense and not for purposes oflimitation, the scope of the inventionrbeing set forth in the followingclaims.

1 claim:

l. An ion source comprising in combination a hollow member at least apart of which is permeable to a gas and having at least one point on itsexternal surface, means to compress said gas into said hollow member,and means to produce an electric ield at said point vof suicientstrength to cause ionization of said gas in the Yvicinity of said point.

2. A positive-ion source in accordance with claim 1, wherein thestrengthV of said electric eld is at least of the order of l03 volts percentimeter, and wherein said electric lield is so oriented as to attractpositive ions from said point.

3. A negative-ion source in accordance with claim l, wherein thestrength of said electric held is at least of the order of 107 volts percentimeter, and wherein said electric lield is so oriented as to attractnegative ions from said point.

4. An ion source in accordance with claim l, wherein said hollow membercomprises a hollow tube terminating in a hollow needle the lateral wallVofY which is permeable to said gas.

5. An ion source in accordance with claim 1, wherein said hollow membercomprises a hollow tube terminating p in a solid needle which ispermeable to said gas.

6. An ion source in accordance with claim l, wherein said hollow membercomprises a hollow tube terminating in a needle having a shaftVpermeable to said gas and having a hard point.

7. An ion source comprising in combination a hollow member at least apart of which is permeable to a gas and Vhaving at least one point onits external surface, means Y to compress said gas into said hollowmember, an electrode spaced from said point, and means to impress apotential difference between said point and said electrode of sufficientmagnitude to cause ionization ofV said gas .in the vicinity of saidpoint. Y

8. A positive-ion source in vaccordance with claimr7, wherein theVradius of curvature of Vsaid point in centimeters is ofthe order of .2times said potential difference in volts divided by the electric iieldin volts per centimeter at said point, wherein said'electric eld isatleast of the order-of 108 volts per centimeter, and wherein said pointis at a positive potential with respect to said electrode. Y Y

9. A negative-ion source in accordance with claim 7,

- wherein the radius of curvature of said point in centimeters is of theorder of .2 times said potential dierence inrvolts divided by theelectric field in volts per centimeter at said point, wherein saidelectric eld is at least of the order of l0" volts per centimeter,andrwherein said point is at a negative potential with respect tosaidelectrode.

An ion source comprising in combination: a hollow member at least a partof which is permeable to a gas and having at least one point on itsexternal surface,

means to compress said gas into said hollow member, means to produce anelectric field at said point of sufficient strength to cause ionizationof said gas in the vicinity of said point, and means to heat said point.

ll. An ion source comprising in combination a hollow member at least apart of which is permeable to isotopes of hydrogen and having at leastone po-int on its external surface, means to compress at least oneisotope of hydrogen into said hollow member, and means to produce anelectric iield at said point of suiiicient strength to cause ionizationof said isotope of hydrogen in the vicinity of said point.

12. An ion source in accordance with claim l1, wherein the permeablepart of said hollow member is composed of palladium.

13. An ion source in accordance with claim 11, wherein the permeablepart of said hollow member is composed of iron and wherein means isprovided to heat the permeable part of said hollow member.

i4. An io-n source in accordance with claim ll, wherein the permeablepart of said hollow member is composed of nickel and wherein means isprovided to heat the penreable part of said hollow member.

l5. An ion source comprising in combination: a hollow member at least apart of which is permeable to a gas and having at least one point on itsexternal surface, means to compress said gas into said hollow member,electrode 'spaced from said point, and means to apply voltage pulses ofshort duration between said point and said electrode of suliicientmagnitude to cause ionization of said gas in the vicinity of said pointduring said pulse.

16. An ion source comprising in combination: a hollow member at least apart of which is permeable to isotopes of hydrogen and having at leastone point on its external surface, means to compress at least oneisotope of hydrogen into said hollow member, means to produce anelectric eld at said point of sufficient strength to cause ionization ofsaid iso-tope of hydrogen in the vicinity of said point, and means forfocusing into a well-collimated beam the ions which are pulled from saidpoint by said electric field.

17. An ion source comprising in combination: a hollow member at least apart of which is permeable to a gas and having at least one point on itsexternal surface, means to compress said gas into said hollow member, afirst electrode spaced from said point, means to impress a potentialdiierence between said point and said iirst electrode of suicientmagnitude to cause ionization of said gas in the vicinity of said point,a second electrode in the vicinity of said point, means to impress aVoltage diierence between said point and said second electrode of amagnitude small relative to the potential difference between said pointand said tirst electrode, and means to vary said voltage dilerence so asto control the emission of ions from said point.

18. An ion source comprising in combination: a hollow member at least apart o which is permeable to a gas and having at least one point on itsexternal surface, means to compress said gas into said hollow member, afirst electrode spaced from said point, means to impress a potentialdiierence between saidvpoint and said lirst electrode of sufficientmagnitude to cause ionization of said gas in the vicinity of said point,a second electrode in the vicinity of said point, and means to impressvoltage pulses between said point and said second electrode of amagnitude small relative to the potential difference between said pointand said irst electrode.

19. An ion source comprising in combination: a hollow tube terminatingin a needle at least a part of which is permeable to a gas, means tocompress said gas into said tube, an electrode spaced from the tip ofsaid needle, means to impress a potential diierence between said needleand said electrode of suicient magnitude to cause ionization of said gasin the vicinity of the tip of said needle, an apertured platesurrounding said needle in the vicinity of the tip thereof, means toimpress a voltage ditte-rence between said needle and said aperturedplate of a magnitude small relative to the potential difference betweensaid needle and said electrode, and means to vary said voltagedifference so as to control the emission of ions from the tip of saidneedle.

References Cited in the iile of this patent UNITED STATES PATENTS2,287,620 Kallmann lune 23, 1942 FOREIGN PATENTS 697,105 Great BritainSept. 16, 1953

